Summary: `echo` correctly interpretes \n on mac, but not on linux. On linux you have to give it `-e` to interpret \n. Unfortunately, `-e` options is not available on Mac. Go back to old way of checking gflags
Test Plan: build_tools/build_detect_platform on mac and linux
Reviewers: sdong
Subscribers: dhruba
Differential Revision: https://reviews.facebook.net/D37515